Display devices, such as liquid crystal display (LCD) devices, are used in a variety of applications including, for example, televisions, hand-held devices, digital still cameras, video cameras, and computer monitors. Because an LCD panel is not self-illuminating, some display applications may require a backlighting assembly or a “backlight.” A backlight typically couples light from one or more sources (e.g., a cold cathode fluorescent tube (CCFT) or light emitting diodes (LEDs)) to the LCD panel.
Common display devices usually include polarizers. The most commonly used type of a polarizer is a dichroic polarizer. Dichroic polarizers are made, for example, by incorporating a dye into a polymer sheet that is then stretched in one direction. Dichroic polarizers can also be made by uniaxially stretching a semicrystalline polymer such as polyvinyl alcohol, then staining the polymer with an iodine complex or dichroic dye, or by coating a polymer with an oriented dichroic dye. These polarizers often have an extinction ratio (the ratio of transmission of light polarized perpendicular to the stretch direction to the transmission of light polarized parallel to the stretch direction) of 1000:1 or more. Dichroic polarizers normally have a substantial amount of absorption of light polarized along the high transmission direction. Many commercial polarizers typically use polyvinyl alcohol as the polymer matrix for the dye, however, other polymers can be used. For example, U.S. Pat. No. 4,756,953 describes the use of polyethylene naphthalate as the polymer matrix.
Another common type of a polarizer used in displays is a reflective polarizer. Reflective polarizers tend to be more efficient in transmitting light of the high transmission polarization. This is due to the use of a non-absorbing dielectric stack for polarizing light. These types of polarizers sometimes have defects, such as leakage of light through localized areas of the sheet and incomplete reflectivity of the high extinction polarization over the wavelength region of interest.
In some displays applications, reflective and dichroic polarizers have been combined, as described, for example, in Ouderkirk et. al., U.S. Pat. No. 6,096,375. The combination of the two polarizers provides a high reflectivity of one polarization and high transmission for the perpendicular polarization for light incident on the reflective polarizer side of the combined polarizer, and high absorption and transmission for light of orthogonal polarizations incident on the dichroic polarizer side.